Rice and a variety of other cereal grains may require drying following harvest to enable milling and to provide for storage and subsequent processing without spoilage.
Primitive drying techniques for rice variously included a simple prolonged exposure of the newly harvested grain to the atmosphere or exposure to warm air produced in a fire pit. Such processes typically result in breakage of a very large proportion of the individual kernels. Shattering of the rice may occur during the drying process itself or may take place later during subsequent handling of the grain as a result of fissures or checks produced in individual kernels during the drying operation.
More advanced rice drying techniques reduce the extent of such breakage and thereby provide a more desirable higher quality product. Under current processing procedures, the harvested rice is passed through a drying tower or towers in which it is repetitively subjected to controlled flows of dry heated air. Heretofore successive exposures to the drying airflow have been separated by tempering periods of at least several hours duration during which the drying airflow is absent. Typically the freshly harvested bulk rice is given one pass through a drying tower and then transferred to a tempering bin where it is stored for periods ranging from several hours to a day or more after which the tempered rice is again passed through the same drying tower or another one. The sequence of drying followed by tempering may be repeated a number of times depending on the particular type of rice, moisture content, quality of product to be produced and other factors.
The forced migration of moisture from the interior of the rice kernels to the surface during a period of drying is inherently a slow process. Consequently the surface regions of the kernels are dried more strongly than the interior and a pronounced moisture gradient builds up within each kernel after a period of partial drying. The drying action then becomes less efficient. The tempering periods allow the moisture to redistribute itself within individual kernels thus reducing the moisture gradients. This enables the next period of partial drying to be more effective until such time as the gradients again build up and another period of tempering is needed.
Tempering bins are a costly and bulky addition to the drying installation. In some more recently developed drying facilities, the rice or other grain is traveled through a drying tower in which both the drying steps and the tempering steps are performed, the grain being subjected to a drying flow of warm air at vertically spaced apart regions within the tower which are separated by tempering zones at which there is no significant airflow.
In either type of installation breakage of a substantial portion of the kernels continues to occur either during the drying process or later as a result of fissuring or checking of the kernels brought about during the drying operations. The broken kernel component of the product must then be separated out and sold at a substantially lower price than the whole grain component. The fissuring and consequent breakage of kernels which occurs in the course of currently practiced rice drying techniques very significantly reduces the overall quality of the product and adversely affects the economic return to the growers, processors and marketing organizations.
In addition to the moisture gradient which is built up in the rice kernels following a period of partial drying a substantial thermal gradient is also created. It has heretofore been the assumption in the industry that fissures in rice kernels, which lead to immediate or later breakage, are a result of thermal stresses during drying. Consequently it has also been assumed that the conventional tempering periods are needed not only to allow reduction of moisture gradients but also to reduce heat stress in order to increase the yield of whole kernel rice. As will hereinafter be discussed in more detail these assumptions appear to be erroneous. I have discovered that the undesirably high incidence of kernel breakage under prior practice can be attributed, at least primarily, to a distinctively different cause. As has been pointed out, the prior processing techniques do not in fact avoid a very substantial incidence of fissuring.
Adverse characteristics of rice drying techniques as currently practiced are not limited to the undesirably low yield of whole kernels. It has been pointed out that tempering bins, where they are required, constitute a costly complication in the drying installation and also complicate the handling of the grain. The long periods needed for tempering reduce the rate at which any given installation can process grain. As a practical matter this may have the indirect effect of causing undesirable delays in harvesting in any given rice growing locality. Drying should be performed quickly once the rice has been harvested.
While the background of the invention has been herein discussed with specific reference to the processing of rice, similar problems are sometimes encountered in the processing of other starchy cereal grains such as wheat, barley, corn or oats, among other examples.
The present invention is directed to overcoming one or more of the problems discussed above.